pubsys: validate-repo without tokio runtime

[webern]

Issue number:

Description of changes:

We were seeing pubsys validate-repo hang and we believe it was because of multiple tokio runtimes. Although it's not clear why tokio::spawn was working before, re-implementing this to use std::thread::spawn instead has fixed the problem.

Testing done:

Created an Infra.toml like this:

[repo.2020-07-07]
metadata_base_url = "https://updates.bottlerocket.aws/2020-07-07/"
targets_url = "https://updates.bottlerocket.aws/targets/"
root_role_url = "https://cache.bottlerocket.aws/root.json"
root_role_sha512 = "90393204232a1ad6b0a45528b1f7df1a3e37493b1e05b1c149f081849a292c8dafb4ea5f7ee17bcc664e35f66e37e4cfa4aae9de7a2a28aa31ae6ac3d9bea4d5"

And this a few times:

cargo make validate-repo \
    -e PUBLISH_REPO=2020-07-07 \
    -e BUILDSYS_VARIANT=aws-k8s-1.15 \
    -e BUILDSYS_ARCH=x86_64 \
    -e PUBLISH_INFRA_CONFIG_PATH="${INFRA_TOML}"

I also edited a target in a repo and verified that the program errors out when the repo is bad:

Failed to validate repository: Failed to download and write target 'migrate_v1.0.6_metricdog-init.lz4': Hash mismatch for https://somewhere.amazonaws.com/somewhere/targets/17a0a6e4f95471734b77091212ccabf564d3312062fc98c995002df2fbf7c6db.migrate_v1.0.6_metricdog-init.lz4: calculated 2c26b46b68ffc68ff99b453c1d30413413422d706483bfa0f98a5e886266e7ae, expected 17a0a6e4f95471734b77091212ccabf564d3312062fc98c995002df2fbf7c6db

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[tjkirch]

LGTM. I'd like to see a test confirming that bad cases still fail as we expect, though, ideally. ✔️